Acoustic measurements of boundary layer turbulence, sediment concentration and sediment flux using the Bistatic Coherent Doppler Velocity and Sediment Profiler (BCDVSP)

Abstract

A pulse-to-pulse coherent acoustic doppler using a bistatic transducer configuration has been developed to study oceanic boundary layers. The instrument uses a central narrow-beam transceiving transducer, with three surrounding fan-beam response receiving transducers directed toward the narrow volume ensonified by the central transducer. Short acoustic pulses ensonify approximately 1-cm-long sections of the water column below the 2-deg-width transmitter transducer, and acoustic energy reflected from scatterers in the water column are received by all four transducers. Coherent sampling methods provide high resolution doppler frequency measurements. The four doppler estimates allow three component velocity vectors to be estimated concurrently with the backscatter amplitude in the same small volumes over a 75-cm range, at 1-cm resolution and 20-Hz sample rate. Laboratory calibration of the acoustic backscatter using in situ sand samples provides sediment concentration profiles over a 0.001 to 25 g/l concentration range. The BCDVSP has been used in two ONR studies of the oceanic bottom boundary layer (BBL) on the inner shelf. Examples of BBL turbulent structure and sediment concentration will be shown from the SandyDuck near shore experiment in 1997, and from two inner shelf sites during the Shoaling Waves Experiment.